CLASS messages are usually transmitted over a voice transmission path during the silent interval between the first and second power ringing signals, to customer premises equipment (CPE). The transmission scheme used is analog, phase-coherent frequency shift keying (FSK) with logical 1's (marks) modulated by 1200.+-.12 Hz and logical 0's (spaces) modulated by 2200.+-.22 Hz. This binary data is sent serially in an asynchronous way at a rate of 1200 bits per second (1200 baud).
Typically, a peripheral of the CPE receiving the data uses a digital to analog (D/A) converter called a CODEC. CODECs which sample analog signals at a fixed rate of 8,000 Hz are relatively inexpensive and are consequently preferred over more complex CODECs having selectable sampling rates. The output of the CODEC is a compounded 8 bit value which could be expanded (in hardware or software) to a linear value with one sign bit and 12 bits (for Europe) or 13 bits (for North America) for magnitude.